The invention relates to an apparatus for degassing an aqueous suspension containing pulp fibers, the apparatus comprising an elongated horizontal reservoir, a bypass manifold that extends into the reservoir from one end thereof and that is parallel to the reservoir, several successive injection pipes originating in the bypass manifold for supplying an aqueous suspension into the reservoir beginning from the inlet end of the bypass manifold over a section of the reservoir""s length, a discharge pipe situated near the opposite end of the reservoir for removing the aqueous suspension from the reservoir, means for keeping the surface of the aqueous suspension substantially at a constant level in the reservoir, and an overflow pipe for returning into circulation additional aqueous suspension that has entered the reservoir.
In a paper machine, pulp fed into the head box and dilution water used for adjustments of the head box must be deaerated as well as possible before the aqueous suspensions are passed into the head box in a paper machine, board machine or some other similar web-forming machine. Usually, the fiber consistency of the pulp in the head box varies from 0.5 to 1.5% and the consistency of the dilution water varies from 0.2 to 0.8%, even though the values may also be different. The variation in the consistency should also be reduced, which is implemented in this kind of apparatus by spraying an aqueous suspension around the reservoir, whereupon the suspension is mixed more evenly since it consists of several separate streams. When an aqueous suspension is supplied to the reservoir via separate vertical injection pipes, the suspension gushes into the upper surface of the reservoir and flows down into the aqueous suspension contained in a basin provided in the reservoir, thus improving deaeration. The aqueous suspension flows in the longitudinal direction of the basin towards a discharge pipe provided at the opposite end of the basin with respect to the injection pipes, and the suspension is passed via the discharge pipe to the head box. Since a sub-atmospheric pressure prevails in the reservoir, air and other gases can be easily removed from the aqueous suspension due to the sub-atmospheric pressure both during the spraying and later from the surface of the suspension. In prior art arrangements, a vertical wall placed at the other end of the basin limits the surface level of the aqueous suspension such that it remains substantially constant, and possible additional aqueous suspension flows over the wall and out of the basin via a discharge pipe provided on the other side of the wall. Such arrangements are disclosed for example in Finnish Patents 63613 and 100950. There are also known arrangements where a bypass manifold extends into the reservoir from one end thereof, and injection pipes placed on the upper surface and on the sides of the manifold are used to spray an aqueous suspension into the reservoir.
In the prior arrangements, a typical problem is that the flow of the aqueous suspension in the basin provided in the reservoir varies greatly in the longitudinal direction of the reservoir. This is due to the fact that closer to the end of the reservoir the amount of the aqueous suspension is smaller with respect to the cross-sectional surface of the reservoir, which results in a relatively small flow near the end of the reservoir. Correspondingly, closer to the discharge pipe the total amount of the aqueous suspension sprayed from the injection pipes increases and the flow is the greatest at the injection pipes that are nearest to the discharge pipe. Therefore, at the end of the reservoir farthest from the discharge pipe the flow amounts may be too small, wherefore the aqueous suspension or a part thereof, such as a filler supplied thereto, may flocculate or settle at the bottom of the reservoir and its consistency may differ significantly from the average consistency of the aqueous suspension in the basin. This causes significant changes in the quality of the finished paper or in some other fiber web product, which is not in any way desirable. With a low flow rate the reservoir also gets dirty more quickly.
The purpose of the present invention is to provide an apparatus for degassing an aqueous suspension that avoids the drawbacks of the prior arrangements and provides a relatively even flow of the aqueous suspension in the basin at the bottom of the reservoir, while preventing the reservoir from getting dirty, so that flocculation, sedimentation and variation in consistencies can be minimized as well as possible. The apparatus according to the invention is characterized in that the bypass manifold is positioned at the lower part of the reservoir such that it is situated substantially within the aqueous suspension in the reservoir.
The basic idea of the invention is that a bypass manifold is placed or formed at the lower part of the reservoir towards the discharge pipe from the inlet end of the aqueous suspension, such that the manifold is situated at least primarily within the aqueous suspension wherefore it decreases the cross-sectional area of the reservoir at the inlet end and thus increases the flow rate of the aqueous suspension. Therefore, the ratio of the amount of the aqueous suspension flowing in the basin and of the cross-sectional area of the flow can be adjusted better, so that the flow rate of the aqueous suspension from the end of the reservoir to the discharge pipe can be made almost constant. According to a preferred embodiment of the invention, the bypass manifold tapers off from the end of the reservoir towards the discharge pipe, and therefore the flow of the aqueous suspension inside the manifold and correspondingly the flow in the reservoir can be adjusted, such that at each point the amount of the flowing aqueous suspension and the cross-sectional area are substantially in a constant ratio. The basic idea of another preferred embodiment of the invention is that the upper surface of the bypass manifold is substantially planar, and the injection pipes can therefore be connected thereto accurately and precisely so that the fastening comprises no unevenness or projections where fibers might accumulate, thus forming flocs or deteriorating otherwise the quality of the aqueous suspension.
The invention has an advantage that at best the flow of the aqueous suspension can be kept substantially constant along the entire length of the basin for aqueous suspension in the reservoir. Another advantage of the invention is that it is easy and simple to implement.